A hydrocarbon-fueled engine may combust a mixture of air and fuel to drive mechanical equipment and perform work. The hot exhaust gas generated by the engine generally contains unwanted gaseous emissions and possibly some suspended particulate matter that may need to be converted to more innocuous substances before being discharged to the atmosphere. The gaseous emissions primarily targeted for removal include carbon monoxide, unburned and partially burned hydrocarbons (HC's), and nitrogen oxide compounds (NOX) comprised of NO and NO2 along with nominal amounts of N2O. An exhaust aftertreatment system that includes specially catalyzed flow-through components may be employed to dynamically treat a continuous exhaust flow with variable concentrations of these emissions. Many different exhaust aftertreatment system designs have been developed. But in general these systems seek to oxidize both carbon monoxide and HC's (to carbon dioxide and water) and reduce NOX (to nitrogen and water). Suspended particulate matter, if present, is usually captured by a filter and burned off at regular intervals.
The catalytic conversion efficiency of carbon monoxide, HC's, and NOX over various types of catalysts depends largely on the air to fuel mass ratio of the mixture of air and fuel fed to the engine. A stoichiometric mixture of air and fuel (air to fuel mass ratio of about 14.7 for standard petrol-based gasoline) combusts to provide the exhaust flow with a reaction balance of oxidants (O2 and NOX) and reductants (CO, HC's, and H2). This type of exhaust flow composition is generally the easiest to treat. A conventional three-way-catalyst (TWC) that includes a platinum group metal mixture dispersed on a base metal oxide support material, for example, can simultaneously reduce NOX and oxidize carbon monoxide and HC's through various coupled catalytic reactions. But a stoichiometric mixture of air and fuel is not always maintained or even practical (i.e., a diesel engine). The engine may, for instance, combust a lean mixture of air and fuel (air to fuel mass ratio above 14.7 for standard petrol-based gasoline) to achieve more efficient fuel economy. The excess air contained in a lean mixture of air and fuel increases the concentration of uncombusted oxygen and decreases the concentrations of the various reductants in the exhaust flow. The catalytic reduction rate of NOX to N2 is slowed in such an oxidative environment over a conventional TWC and may require an entirely different system design or supplemental NOX treatment capacity to decrease NOX concentrations to acceptable levels.
The two most prevalent approaches, to date, for reducing NOX in an oxygen enriched exhaust flow are a selective catalytic reduction (SCR) system and a lean NOX trap (LNT). A SCR system introduces a reductant such as ammonia or a hydrocarbon into the exhaust flow which, in turn, reacts with NOX in the presence of oxygen over a reaction-specific SCR catalyst to form nitrogen. A LNT directs the exhaust flow over a NOX absorption catalyst that stores NOX as a nitrate species until purged with a source of reductants that also converts the absorbed NOX into nitrogen over a NOX reduction catalyst. The overall NOX conversion efficiency for both practices can be enhanced by decreasing the molar ratio of NO to NO2 in the NOX constituency originally produced by the engine and contained in the exhaust flow. The NOX gas constituency generated by the engine, when combusting a lean mixture of air and fuel, generally constitutes greater than 90 mol % NO and less than 10 mol % NO2. A preferred NO/NO2 molar ratio for rapid NOX reduction is approximately 1.0 (equimolar). For this reason, an oxidation catalyst that promotes NO oxidation to NO2 is usually provided within or in front of the SCR catalyst or the NOX absorption catalyst to boost NOX reduction activity in the SCR system or the LNT, respectively.
Perovskite oxides are a broad class of non-noble mixed metal oxide compounds that can facilitate NO oxidation. The crystalline lattice of perovskite oxides can accommodate different lattice defects that often create oxygen vacancies. These oxygen vacancies, without being bound by theory, are believed to contribute significantly to the perovskite oxide's NO oxidative activity as oxygen contained in the exhaust flow disassociates to fill those vacancies leaving behind residual oxygen radicals that quickly attack NO. Perovskite oxides are defined generally by an ABO3 crystal structure in which a larger, centrally located “A” cation and smaller, surrounding “B” cations coordinate with twelve and six oxygen anions, respectively. Small amounts of the “A” and “B” cations may be substituted with different yet similarly sized “A1” and “B1” promoter cations to provide a supercell crystal structure similar to the general ABO3 crystal structure but defined by the formula A1-XA1XB1-YB1YO3. Several specific perovskite oxides that have shown promise as an oxidation catalyst—taking into consideration NO oxidation activity and hydrothermal durability—are LaMnO3 and strontium and cerium promoted supercell variations of that perovskite oxide having the general formulas La1-XSrXMnO3 and La1-XCeXMnO3, respectively, with X ranging from 0.01 to 0.50 in each instance. A more in-depth discussion of perovskite oxide catalysts can be found in Arendt et al., “Structuration of LaMnO3 perovskite catalyst on ceramic and metallic monoliths: Physico-chemical characterization and catalytic activity in methane combustion,” Applied Catalysts, A: General 339 (2008), pp. 1-14.
Perovskite oxides including LaMnO3 and its promoted derivations may be dispersed as fine particles and optionally supported on a base metal oxide or some other suitable support material to optimize the oxide's accessible surface area and achieve the most effective oxidation of NO to NO2. A variety of preparation methods have been developed that purport to maximize the oxidative catalytic activity of perovskite oxide particle dispersions. Research into perovskite oxides and their methods of preparation is nonetheless still ongoing and, in fact, has picked-up as the catalyst and exhaust treatment industries seek alternatives to the expensive platinum group metals that have conventionally been used to formulate oxidation catalysts.